We stayed at the same campsite as mentioned above last year (
http://www.eaurouge.nl/) - arriving in the pouring rain on the Friday. Camping was very cheap and the facilities weren't bad. The atmosphere was friendly with a great little bar/terrace out front to enjoy the Belgian beer. We left our tents for the entire race (staying at the circuit the whole night) and returned on the Sunday afternoon and all our equipment had remained undisturbed - nothing nicked. A big advantage of staying at this site is that you have showers and toilets - most of the others are just camping in a field with the odd portaloo dotted about.
You can walk to the circuit from the site (or cycle). We bought a parking pass for the lot near Eau Rouge (can be purchased at the welcome centre on the left as you approach the village of Francorchamps). One of the best things about Spa is that you can walk round the entire circuit, giving plenty of 'catch-fence-free' photo opportunities - just make sure that you take waterproofs and an umbrella as it is virtually guaranteed to rain!!
Can't make it myself this year as the missus is due to pop sprog no2 around this time (bad planning on my part me thinks) but to any of you who have not been and are thinking of going - just do it - you will definitely enjoy a great race weekend at what is one of the best tracks in the world.